The goal of the proposed research is to understand specific ionic and molecular mechanisms that act at the cellular level in the vertebrate retina during light- and dark-adaptation. Photic stimulation of the retina causes rod photoreceptors to remove K+ from the extracellular space, leading to a decrease in [K+]o. The Muller (glial) cells are likely to be affected directly by this light-evoked decrease in [K+]o. Moreover, the changes in [K+]o are likely to be accompanied by changes in the movements other important ions, such as Na+ and C1-, across cell membranes. The resulting changes in intracellular and extracellular ionic concentrations will affect rod function directly, and will affect Muller cell function indirectly. These ionic movements and interactions will be examined in the isolated retina of the toad, Bufo marinus. Experiments will identify ionic mechanisms that produce light-evoked changes in ionic concentration, and will determine the effects of these light-evoked changes in concentration upon rods and Muller cells. These experiments will involve intracellular measurement of membrane voltage, in combination with intra- and extracellular measurement of ionic concentrations using ion-selective microelectrodes, both in darkness and during periods of maintained illumination. Within rods, many biochemical reactions must participate in the transduction and adaptation processes. Experiments will use electrophysiologial methods to assess the role of certain of these biochemical reactions in the control of rod photoresponses. Proteins and other molecules that are thought to participate in these reactions will be pressure-injected into rods while recording membrane voltage. These experiments will help to elucidate molecular mechanisms involved in the biochemical control of electrophysiological responses in rods. The proposed experiments will provide basic knowledge of ionic and molecular mechanisms that function at the cellular level in rods and in Muller cells. Knowledge of these mechanisms may lead to better understanding of disease processes that disrupt function in the outer retina. Experiments that examine ionic mechanisms in Muller cells will help to determine the cellular origin of several components of the electro-retinogram (ERG), which may increase the diagnostic usefulness of the clinical ERG.